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Electronic and structural properties of copper monoxide (CuO) sintered as a common ceramic and nanoceramic are studied by positron annihilation spectroscopy. A CuO nanoceramic with crystallite size ranging from 15 to 90 nm was prepared from a common one by shock-wave loading. It is found that the momentum distribution of valence electrons in CuO is shifted, as compared with metallic copper, towards higher momentum values. This result is related to the effect of the Cu 3d–O 2p hybridization in the Cu–O ionic covalent bond formation. It is found that open volumes, identified mainly as small agglomerates of oxygen vacancies, appear at the nanoceramic crystallite interfaces. The degree of the Cu–O bond covalency decreases locally at the crystallite interfaces because of an oxygen deficit. The nanocrystalline state in CuO is shown to be thermally stable up to 700 K.